1. Field of the Invention
The present invention relates to a method for obtaining combustion gases of high calorific value.
2. Description of Related Art
Careful use of resources becomes more and more the central objective of society. Energy generation from waste materials and regenerative substances such as biogenic fuels during first or consecutive use is thus of special importance. Furthermore, towards the end of the 20th century the generation of hydrogen becomes more and more the center of interest, not least due to the beginning exploitation of hydrogen in fuel cells.
The energetic exploitation of solid, paste-like or liquid fuels is most of the time carried out by way of combustion with subsequent use of the previously chemically bound heat released during combustion.
Apart from this, there have been approaches for a long time to establish gasification processes for generating combustion gases of high calorific value from solid, paste-like or liquid fuels. The combustible part of the crude gas during each gasification consists for the greatest part of hydrogen and carbon monoxide; smaller amounts are methane ad higher hydrocarbons. Each type of gasification thus generates hydrogen.
An essential advantage of gasification over combustion is that the pollutants contained in the starting substance are converted in a reducing atmosphere into constituents or into relatively simple chemical compounds. The gas volumes are considerably smaller in comparison with combustion, so that gas purification in the case of gasification can be carried out more easily and at lower costs as compared to combustion when the objective is the same.
There are three basic types of gasification methods:    1. Gasification of solid, paste-like or liquid fuels with the gasification medium air is in technical terms the simplest method and leads to partial oxidation. The calorific value of the gas produced thereby is lower than that of the fuel used. The gasification temperatures are typically within the range of 600° C. to 900° C. Tars are produced at said temperatures to a considerable extent. A large-scale use of the method has so far not been possible because so far the removal of tars from the gas could not be sufficiently controlled technically for small gasifiers.    2. Like air gasification, the gasification of solid, paste-like or liquid fuels with the gasification medium oxygen results in partial oxidation with a decrease in the calorific value. The gasification temperatures are typically at 1600° C. so that the formation of tar is ruled out. A large-scale use has so far not been possible because the generation of the necessary oxygen entails high costs and excessively burdens economic calculations in industry. In comparison with air gasification, oxygen gasification leads to smaller gas amounts because the gasification medium does not introduce an inert nitrogen amount    3. The gasification of solid, past-like or liquid fuels with the gasification medium steam leads to a gas of a higher calorific value than the fuel used originally. Therefore, heat must be supplied to the gasification reactor from the outside. The gasification temperatures are typically between 600° and 900° C. Tar might be formed. However, its potential is lower than in air gasification. A large-scale use has so far not been possible because the problem of heat input into the reactor has, in particular, not been solved in a satisfactory way. The gas amounts of the steam gasification lie between those of air and oxygen gasification. This is due to the fact that during steam gasification the carbon of the fuel is oxidized by the oxygen of the steam into carbon monoxide or carbon dioxide, whereby additional hydrogen is formed. The potential of the steam gasification to generate hydrogen is thus considerably higher than that of air or oxygen gasification.
Gasification methods in which the reaction heat needed is supplied by partial oxidation are called autothermic, whereas those in which the reaction heat needed is supplied from the outside are called allothermic.
The allothermic steam gasification of solid, paste-like or liquid fuels normally takes place in a fluidized bed for ensuring uniform reaction conditions. In this process, steam flows from below to a bed of small solid particles. The inflow rate is here so high that the solid particles are at least kept suspended. One talks about a stationary fluidized bed when the solid particles form a fixedly defined surface with ascending gas bubbles, whereas in a circulating fluidized bed the main part of the solid particles is discharged with the gas flow from the fluidized bed reactor and is separated from the gas flow and then supplied again via a down path to the lower part of the fluidized bed reactor proper. The solid particles may be inert, consisting e.g. of quart sand, limestone, dolomite, corundium, or the like, but they may also consist of the ash of the fuel. The solid particles can accelerate the gasification reactions due to catalytic properties.
The Nack et al U.S. Pat. No. 4,154,581 describes a gas generator comprising two reaction zones and having an exothermic reaction environment in the heating portion, so that heat is directly provided. Heat transportation is ensured by using bed material of different grain sizes. A coarse-grained material remains in the exothermic bed, whereas a fine-grained fraction travels from the exothermic into the endothermic region and back. The fine-grained fraction assumes the function of heat transfer.
Said method has the drawback that the transportation of the solids between the beds must coincide with the heat balance of the beds, which makes great demands on the control units at high working temperatures and different load conditions. Furthermore, as far as the fuels are concerned, there is no Separation between the combustion region and the gasification region, so that possible pollutants from the fuel may be found along both the gasification path and the combustion path, which complicates the gas cleaning system.
It is known from the European patent No. EP 0 329 673 and the U.S. Patent to Mansaur et al U.S. Pat. No. 5,059,404 that heat input is realized with the help of heat exchangers which are provided in the fluidized bed, i.e. in the reaction zone. The drawback of such a concept is that the arrangement of the heat exchangers in the reaction zone predetermines the dimension of the reaction zone and the fluidized bed, respectively, because of the heat exchange surfaces required. Moreover, the heat exchange surfaces are directly exposed to the corrosive effects of harmful constituents of the fuel, which makes extreme demands on the material at surface temperatures of from 600° C. to more than 900° C. Finally, a combination of autothermic and allothermic methods is known from the German patent No. DE 197 36 867 A1. The necessary reaction heat is here supplied via hot steam and flue gases from a partial combustion of the product gas.
The combination of an autothermic and allothermic method has the effect that the gas amount increases considerably due to the nitrogen amount which is supplied with the air for partial combustion. Thus the partial pressures of the industrial gases decease, which has a negative effect on the subsequent gas cleaning and the after treatment of the gas.
A fluidized bed constitutes a technology which has been tried and tested and often employed for many years. Applications are e.g. the drying and burning of solid materials or of slurries. The basis for each fluidized bed method is a reactor in which a solids content is loosened by inflow from below to such an extent that the individual particles start to float in air, with the solids content being fluidized.
A distinction is made between two coarse types: When a solid surface of the fluidized solids content is formed, one talks about a stationary fluidized bed. When the particles are discharged with the gas flow from the reactor, one talks about a circulating fluidized bed. Further essential feats of every circulating fluidized bed are an apparatus for separating the discharged solid particles from the gas flow and a further means for returning the separated solid particles into the reactor.
In the course of time many constructional forms have been used for both basic types in the attempt to avoid the drawbacks of the one type and to exploit the benefits of the other.
The following documents should be mentioned by way of example:    DE 28 36 531: A stationary fluidized bed method in which regions of different fluidization are formed by installing a partition, so that bed material is circulated in a stationary bed.    EP 0 302 849: A circulating fluidized bed which develops DE 28 36 531, but rather reminds of a stationary than a circulating fluidized bed because of its constructional size.    DE 33 20 049. A stationary fluidized bed method in which bed material is circulatd due to different bed heights.